Don't pass "" to cros_run_vm_test if vm_graphics_flag isn't set.
Don't pass "" to cros_run_vm_test if vm_graphics_flag isn't set.

This would not have been a problem if "--no_graphics" is used.

Change-Id: I3645391d25d4bb8618109ff5be0ca2fbe8e96a60

BUG=chromium-os:9373
TEST=Ran by hand

Review URL: http://codereview.chromium.org/5104006

bcwaldon/coreos-scripts,vmware/coreos-scripts,zhang0137/scripts,mischief/scripts,smilart/scripts,nicescale/scripts,trnubo/scripts,nicescale/scripts,vmware/coreos-scripts,sigma/coreos-scripts,crawford/scripts,trnubo/scripts,mjg59/scripts,gtank/scripts,BugRoger/scripts,fivethreeo/scripts,marineam/coreos-scripts,crawford/scripts,fivethreeo/scripts,andrejro/coreos--scripts,smilart/scripts,coreos/scripts,nicescale/scripts,sigma/coreos-scripts,crawford/scripts,glevand/coreos--scripts,fivethreeo/scripts,zhang0137/scripts,coreos/scripts,vmware/coreos-scripts,gtank/scripts,mjg59/scripts,BugRoger/scripts,marineam/coreos-scripts,BugRoger/scripts,glevand/coreos--scripts,bcwaldon/coreos-scripts,mischief/scripts,mischief/scripts,endocode/scripts,gtank/scripts,zhang0137/scripts,sigma/coreos-scripts,trnubo/scripts,andrejro/coreos--scripts,endocode/scripts,smilart/scripts,bcwaldon/coreos-scripts,marineam/coreos-scripts,andrejro/coreos--scripts,endocode/scripts,mjg59/scripts